The invention relates to electrostatically produced structures, for example, tubular vascular grafts, and to methods of manufacturing such structures.
It has been proposed in the past to spin electrostatically fibrous structures of polymeric material such as polyurethane using an electrostatically charged, spinning mandrel as a fiber collector, a solution of the polymer being ejected towards the mandrel from a manifold of capillary needles or other suitable means. The mandrel may be replaced by an alternative electrostatically charged collector if a tubular structure is not required.
It has been found that synthetic fibrous structures built up in this way generally have fibers of a diameter not larger than 1 .mu.m and that the fibers are generally randomly oriented. Some directional bias can be induced by varying the speed of mandrel rotation, as indicated in our published British Patent application Nos. 2121286A and 2120946A.
It has been found that the generally random nature of the fibrous structure and the small fiber size of 1 .mu.m or less has produced a tubular structure which can be prone to kinking and can therefore be a problem when used as an arterial graft particularly where limb movement is involved.
In the past, attempts have been made to control the orientation of the fibers as they are deposited on the mandrel. For example, in U.S. Pat. No. 4,689,186 the mandrel is placed between charged electrodes or grids in order to control fiber orientation. As shown and described in U.S. Pat. No. 4,689,186 droplets of fiberizable solution are forced to form on the tips of the manifold needles and are electrostatically drawn towards the charged mandrel. Each droplet is elongated into a thin stream which dries out to form a solid fiber. The path traced by a fiber on its journey towards the mandrel is helical. The radius of the helix is maximum approximately half way between the needles and the mandrel and is minimum at each end of the path. Each needle produces a single fiber, but because the velocity of the fiber is high (30 m/s) and the path is helical, it gives an optical illusion of appearing as a spray or jet, as schematically depicted in FIG. 6(b) of the patent.
While controlling the fiber orientation of a tubular structure can improve many of the physical properties of the structure, it does not eliminate the kinking problem and does not produce a vascular graft that is acceptable for use in all of the desired replacement areas of a natural vessel.